Waveguide rotary joint



Nov. 5, 1957 H. J. RIBLET ETAL WAVEGUIDE ROTARY JOINT Filed Nov. 25, 1955 a m 2 a 2 6. n i s VMM. LWL.-

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United States Patent t WAVEGUIDE ROTARY JOINT HenryI. Riblet, Wellesley, and Theodore S. Saad, West Roxbury, Mass., and Robert L. Williston, Milford, iN.lH., assignors to Microwave Development Labora- `tories', Waltham, Mass., a corporation of Massachusetts Application November-'25, 1953, Serial No. 394,294

8 Claims. `(Cl. S33-98)' 'The present invention relates to waveguide rotary joints for the transmission of microwavestthrough atransmission system capable of angular adjustment. In particular, it relatesto an improved transition v'from rectangular waveguide tocoaxialline which is particularly suited for use in rotary joints.

Theprincipal object of the present invention is to providea' waveguide rotary joint which operates over a wider band of frequencies with a lower voltage standing wave lratio than hasbeen obtainable heretofore.

A further object of the present invention is to provide a waveguide rotary joint which may be used for high power transmission at conditions of low atmospheric pressure, as-forl example, in an airplaneat high-altitude.

Anotherobject ofthe present invention is-to provide a waveguide rotary joint in which the variations of the reection coeiicient and transmission coetcient as a function of'jtherotation-of thefrotaryjoint are negligible.

Otherand further objects of the present invention will appear from the following description.

--In general, the invention contemplates the provision` of a rotar-yjoint comprising a rectangular input waveguide, a` modetransformer for converting a T. E. 1.ofmode existing. in the rectangular input' waveguide tota symmetric T. mode existing inthe-coaxialfline, thus permitting angular rotation, and then retransforming the T. E. M.

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mode to the rectangular T. E.1,o mode in the output rectangular waveguide. This is done by a particular-choice of guiding surfaces which permits the transition vfrom `the .rectangular waveguide to the coaxial lineto be'made with a'very low VV. S. W; R. over a `wide band of frequenc'ies.VV Itis this transition, vthen whichis the content l'ofthe :present invention.

This transition consists first of smooth, specially'tshaped tapenfrom' the rectangular waveguide .havingJa nominal impedancel'ofiabout 400910 /a section lof ridged wavefgude'iwith anominalimpedance of aboutv65tl, and then'a .'novelf'trans'ition from the 4ridged waveguide to a coaxial line having a characteristic impedance of SOQ.

' r:Byfexamining :a large/number fof .ridged waveguide to coaxial .lineftransitiona we have discovered that afright circular cone terminating? in `a: cylinder having' substan- 'ti'ally the same Sdiameter altits ibase as the widthoflthe `ridgeand Vamic-hed theretoV and staperin 'at its :apexl to 'the diameter of the inner conductor of 'the coaxillirie and attachedl thereto to form a, continuation of the inner conductor if projected suthcientl-'yinto abell mouthed opening provided in the' Aouter conductorcofthe 'coaxial line, will result .in a transition having very lowV.-v S. W. Rfs over lbrladfhands Yof frequency if the ridged waveguide-is terminated suitably.y We have found that a suitable reactive termination consists 'of a portion `of an inverted, right circular conical funnel-shaped surface,` having y.atits .base substantially the diameter" of Lthe "baset df the` pyramidal cone which forms an extension of the inner conductor and at its apex substantially the diameter of the outer edge of the bell mouthed opening of the outer conductor 2,812,503 lPatented Nov. 5, 1957 ice of the'coax'ial. line. This funnel shaped termination is a por'fti'o'nfia'right' circular inverted cone. It 'is'p'lacd's'o that its base and apex are inicontact with the correspond- -ingfp'rtinof'the pyramidal cone and bell'rnou'thedv opening `which define its dimensions. This funnel-shaped surface extends part way around the coaxial line'u'ntil its extremities lie in and are part of that plane which is substantially tangent to the inner conductor of vthe coaxial line and is perpendicular to the axis of the waveguide and further from its mouth. The shape of the bellmouthed opening is obtained by rotating a quadrant of -a circle about the axis of the vcoaxial line. This circlev is tangent to the upper wall of the rectangular waveguide at its widest'point, and also tangent tojthe inner surface of the outer conductor of the coaxial line. n

We have found after many experiments that the optimum angle'for thevsurface oyfthe circular cone is in the neighhorhoodof S31/2, while theY angle of the inverted conicalv funnel-shaped `surface is 351/2". Although the precise reasonsfor the surprising performance of this rotary joint is unknownto us,jit isworthy of note that in the' coaxial-region between the pyramid'al cone and the bell-mouth opening to the coaxial line, the diameters are such that in the frequency lband of operation of thefde-vice, it is possible-for the T. E.2,0- mode for coaxial line to propagate as well as the T.Y E. M. mode. For example, in our preferredmodel-the diameter of the outer conductor of the coaxial line-is 1.1122, whilethe diameter of the pyramidal concat -the-corresp'onding positionis .500". 'Thusf the mean diameter is .811. vThis corr'espondstoa cut-0H wavelength of 6.5 cms. Since'theoperating band of this lrotary' joint fallsbetween 3 and 3.7 cms., it isl clear that this 'rnode -can propagate'a 'substantial distance yinto the vbell-mouthed transition of the coaxial line.

vA morecornprehensive understandingof the invention will be afforded by "the following'Y detailedV description considered in conjunctionwith `the accompanying drawings and vthose features'of'the invention which Vare believed to be=novelfandLpat`entable will bepointed'out in thel` claimsappende'd thereto.

In the accompanying drawings, which form part of this specification and which are to be 'read in conjunctiontherewith, and-in which `lilcereferen'c'e numeral'sfare' sed to indicatelike parts in the various views:

Figure rl is a sectional'elevation-ofa kwaveguide rotary jointshowingone 'embodimentof our invention. f

lFig-ure -Z--is aplan view viewedalong the line 2 2 of Figure 1. v

Figure 3 is a sectional view showing? in greater `detail theftr'ansitlion frorn waveguide to'coa'xial llineV along the lin'e @3v-Stof vFigure 2. y

Figure 4-is a" sectional View 'showing in greater detail the transition Yfrom `waveguide 'to coaxial line along the line l 4 4 of Figure A Figure'S is afgraphlg' Y'.ingVfS. W. Rfs obtainable with our "invention over a 'band of frequencies.

vMoreparticularlynow referring to' Figures l-4 the-presentinvention'coniprises-a k'lower'transition [made of va vmetallicalloylpossiblyaluminumv and an essentially similar u'pper transition 1'. Flange's 2'are provided for connecting the rotaryjoint toinpu'twav'eguides. The lower duct -3 "is Irectangular in 'cross )section "and enters into ya section offridiged waveguide iiwhicli is U-:sh'aped in cross section'.

a bell mouthed continuation of the inner surface 11 of the outer conductor 12 of a coaxial line 13. The inner conductor 14 of the coaxial line 13 is attached to the upper portion 15 of the cone `7. The ridged guide 4 beyond the cone 7 terminates in a curved metallic short circuit 16 which is bounded by a portion of an inverted conical funnel-shaped surface which at its upper outer circular edge 17 approximately touches the outermost edge of the bell mouthed opening 8, and at its lower inner circular edge 18 approximately touches the lower edge of the cone 7. This metallic short 16 proceeds substantially symmetrically around the coaxial line 13 until its two extremities 19 lie in a plane which is substantially tangent to the rearmost edge 20 of the inner conductor 14 of the coaxial line 13. The portion 21 of the ridge guide 4 below the short 16 is terminated with metallic plugs 22 which ll the space below the short 16, but may be displaced backward slightly from its extremities 19 away from the coaxial line 13` as shown in Figure 4.

A choke assembly 23-is provided in the outer conductor 12 of the coaxial line 13 which permits rotation of the outer conductor portion of the rotary `joint without introducing any serious mismatch. The details of this choke assembly will not be described in detail, because it is well known to the art. Also shown is a bearing and housing assembly 24 which mechanically supports the two portions of the rotary jointand permits rotation of one portion with respect to the other. The details of this assembly form no part of the present invention and are well known to the art. The free rotation of the inner conductor portion of the two halves of the rotary joint is made possible by the choke 23 which is well known to the art. The

`details of the upper portion of the rotary joint are identical with those of the lower portion except possibly for the choke assembly 23.

The performance of the present rotary joint is remarkable in that very low V. S. W. R.s are obtained over a wide band of frequencies. This has been achieved after extensive experimental effort and can be duplicated only if the dimensions of the components are held to close tolerances. Accordingly the precise dimensions employed or rather their relative magnitudes is an essential part of our invention. Our experimental measurements were made with a unit in which the rectangular waveguide 3 had the dimension of 1.122" x` .497". `The centrally located ridge had a width of .688" and tapered over a length of 1.167" with an S-shaped curve from zero height to a thickness of .344. The cone 7 had an outer diameter of .688 and an angle of S31/2 measured from the surface of the ridge. The height of cone 7 was .328 and it tapered to a diameter of .218" which isalso the diameter of the inner conductor 14. The cone 7 also had ashort vertical cylindrical section 26 which was approximately .050 high. The angle of the surface of the funnel shaped short 16 was 351/ measured from the surface of the ridge. The radius of the circle which defined the bell mouthed opening was .312" with the inner part of the bell mouthed opening tangent to the inner surface of the outer conductor 12 of the coaxial line and tangent at its outer opening to the upper wall 9 of the rectangular waveguide 3. The diameter of the outer conductor of the coaxial line was .500, while the diameter of the inner conductor of the coaxial line was .218. The actual length of the coaxial line was found to be non critical so long as it exceeded a minimum length of about 1".

We have accomplished the objective of my invention, as will be seen from the performance (shown in Figure 5) for a rotary joint constructed in accordance with our invention. When the spacing between the` center lines of the two rectangular waveguides was 3.5" no variation in the electrical performance of the rotary joint could be observed over the frequency band shown.

It will be found that the relative magnitudes as given in the values above should be preserved, that is, for instance, where larger waveguides are used the remaining dimensions should be increased preserving in general the same angles for the elements as herein set forth.

Having now described our invention, we claim:

1. A microwave transmission system comprising, a conductively terminated hollow rectangular waveguide section, a coaxial line section having inner and outer cylindrical conductors perpendicularly extending outwardly from a broad wall of said waveguide` section in the region of said conductive termination, a conductive tapered ridge disposed centrally on the broad Wall of said waveguide opposite said coaxial line section and extending from a point within said waveguide to said conductive termination, said outer cylindrical conductor of said coaxial line being formed with a bell-mouthed section opening into said waveguide opposite said ridge, a conductive conical member based upon said ridge and converging into said coaxial line inner conductor within the region of said bellmouthed opening, said waveguide termination being formed with an arcuate conical `conductive surface inverted with reference to, and partly encircling the base of, said conical member.

2. A microwave transmission system as in claim l and arranged whereby for normal operating frequencies the coaxial region defined between said conical member and said bell-mouthed opening are dimensionally capable of supporting microwave propagation in both T. E. M. and T. E.z,n modes.

3. A microwave transmission system as in claim 2 wherein the width and height of said tapered ridge are 61% and 69% of the width and height respectively of said rectangular waveguide.

4. A `microwave transmission system as in claim 2 wherein the base diameterV of said conical member is substantially equal to theY width of said tapered ridge.

5. A microwave transmission system` as in claim 2 wherein the height of said conical member measured from said .tapered ridge to the juncture with said coaxial line section inner conductor is substantially 96% of the height of said tapered ridge, and further wherein said conical member projects for substantially half its height into said bell-mouthed opening.

6. A microwave transmission system as in claim 2 wherein said arcuate conical conductive surface terminates at one end at the base of said conical member and at the other end thereof at the outer edge of said bell-mouthed opening, the extremities of said conical surface falling in a plane perpendicular to the axis of said waveguide and tangent to the edge of said inner conductor nearest said conductive'termination.

7. A microwave transmission system as in claim 1 wherein. said coaxial line section is formed with a rotary choke joint in a region of said inner and outer conductors displaced axially from said bell-mouthed opening and said conical member.

8. Apparatus permitting angular adjustment between rectangular waveguide components comprising, rst and second substantially like transmission systems each as defined in claim 1J the respective coaxial line sections of said tirst and second transmission systems being coupled by a rotary choke joint.

References Cited in the tile of this patent UNITED STATES PATENTS Great Britain June l5, 1949 

